<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .aa {
            background-color: #2e6da4;
        }
        .aa:hover {
            background-color: red;
        }
    </style>
</head>
<body>
<div class="aa">aaa</div>
</body>
</html>